QTI Snapdragon Display Engine (SDE) writeback display

Required properties:
- compatible:		"qcom,wb-display"

Optional properties:
- cell-index:		Index of writeback device instance.
			Default to 0 if not specified.
- label:		String to describe this writeback display.
			Default to "unknown" if not specified.

Example:

/ {
	...

	sde_wb: qcom,wb-display {
		compatible = "qcom,wb-display";
		cell-index = <2>;
		label = "wb_display";
	};

};
